In October 1957, after several years of successful operation, the workers at Windscale noticed some curious readings from their temperature monitoring equipment as they carried out standard maintenance. The reactor temperature was slowly rising during a time that they expected it to be falling. The remote detection equipment seemed to be malfunctioning, so two plant workers donned protective equipment and hiked to the reactor to inspect it in person. When they arrived, they were alarmed to discover that the interior of the uranium-filled reactor was ablaze.
Windscale's two nuclear piles had been constructed in concrete buildings just outside of the small village of Seascale, Cumbria to produce Britain's bounty of weapons-grade plutonium. The fission reactors had a straightforward air-cooled configuration which allowed each one to exhaust its excess heat through a tall chimney. Breeder reactors such as those at Windscale create plutonium by bombarding the most common isotope of uranium (uranium-238) with neutrons. Any uranium atoms which happen to absorb a neutron briefly become uranium-239, an unstable element which rapidly decays into neptunium-239. Having a half-life of only 2.355 days, this element also soon decays, resulting in the desired plutonium-239.
Each of the heavily shielded Windscale reactors was comprised of a stack of massive graphite bricks. A series of vertical boreholes through these blocks acted as channels for the reactor's control rods which were used to absorb loose neutrons and thereby govern the fission rate. Hundreds of horizontal channels were carved into the blocks in a octagonal pattern for inserting canisters filled with whatever substances the scientists wished to bombard with neutrons. Many contained uranium to convert into plutonium, but others were special isotope cartridges for producing radioisotopes.
The canisters were pushed into place through the front of the reactor-- known as the charge face-- and once the neutrons had worked their magic and turned a good portion of the metallic uranium into plutonium, they were pushed out through the back into a water duct for cooling. The reactor itself was cooled by way of a fan-driven air duct which forced air over the reactor core and out the 400-foot-tall discharge stacks. As a last-minute modification, and at a great effort and expense, a filtering system was added to the top of each chimney at the insistence of a physicist named Sir John Cockcroft. These filters came to be known as "Cockcroft's Folly" due to their engineering difficulty and questionable value.
It was not understood during the plant's construction that graphite which is subjected to neutron bombardment has a tendency to store that energy within dislocations in its crystalline structure. This stored energy is called Wigner energy, named after physicist Eugene Wigner who discovered the effect during his own experiments. Left unchecked, graphite has a tendency to spontaneously release its accumulated Wigner energy in a powerful burst of heat. This was made apparent after two years of operation, at which time unexpected temperature increases were observed in the cores. On one occasion this occurred while the reactor was shut down.
For a time, annealing succeeded in preventing the excessive buildup of Wigner energy. But the reactors and their attendant instrumentation were not designed with annealing in mind, therefore the monitoring equipment tended to provide misleading feedback to the reactor operators. The cycles were also notoriously unpredictable, releasing the pent-up energy at temperatures which varied from one instance to the next. In 1957, Windscale operators modified their procedures to require annealment every 40,000 Megawatt-days rather than every 30,000. They were growing concerned with the observation that higher temperatures were required each time, and that unexpected pockets of excessive Wigner energy were lingering in the graphite piles between cycles.
On 7 October of that year, the operators of Windscale Atomic Pile Number 1 began what would turn out to be its final annealing cycle. After the initial heating of the reactor core, the control rods were re-inserted to slow down the fission process and allow the reactor to cool. The temperature monitors, however, indicated a premature dwindling of temperature in the core, leading the operators to believe that the annealing had not been successfully initiated. Unbeknowsnt to the workers, the readings produced by their equipment were inaccurate due to a combination of improperly placed instruments and uneven heat distribution caused by higher-than-normal pockets of Wigner energy.
Based on this misleading information, the operators made a fateful decision-- they restarted the annealing process by heating the reactor once more. When the control rods were withdrawn to allow the fission reactions to increase, the temperature inside the graphite stack increased to dangerous levels. The heat became so extreme within the core that one of the canisters containing uranium or magnesium/lithium isotopes ruptured, spilling its contents and causing oxidation. The blocks of graphite-- a substance which cannot burn in the air except under extreme conditions-- began to smolder.
Early in the fourth day of the annealing process, operators felt that something was amiss when some instruments indicated the core temperature was not slowly falling as expected, but actually increasing. Their fears quickly compounded as they realized that the needles were pegged on the radiation meters at the top of the discharge stacks. The shift foreman declared an emergency. When the operators attempted to examine the pile with a remote scanner, much to their frustration the mechanism jammed. The reactor manager's deputy Tom Hughes and another operator then made their way to the charge face of the reactor wearing protective gear to make a visual inspection of the core. A fuel channel inspection plug was opened, and as Hughes later recounted, "We saw to our complete horror, four channels of fuel glowing bright cherry red."
Unsure of how to deal with a fire of this nature, operators tried turning the cooling fans to full power in order to bleed off heat, but the oxygen provided by this effort only fueled the fire. Tuohy suggested removing fuel cartridges from the heart of the fire manually by forcing them from their channels and into the cooling ponds using scaffolding poles. The effort was valiant, but the poles were unable to withstand the punishment. They were red hot as as they were withdrawn from the nuclear furnace, and the ends were dripping with molten radioactive uranium. As one of the men battling the unique fire described the exposed fuel channels, "It was white hot, it was just white hot. Nobody, I mean, nobody, can believe how hot it could possibly be."
Next the men borrowed twenty-five metric tons of liquid carbon dioxide from the newly-built gas-cooled Calder Hall reactors next door. Equipment was rigged to deliver the carbon dioxide to the charge face, but the heat from the fire was so intense that the oxygen was liberated from the carbon atoms upon contact, feeding the blaze into a renewed intensity.
By the morning of Friday 11 October, eleven tons of uranium were burning. Equipment was registering temperatures of 1,300 degrees Celsius in the reactor, and climbing at a rate of 20 degrees per minute. The cement containment around the burning reactor was in severe danger of collapse due to the extreme heat. Having no other viable options, the operators decided to attempt to extinguish the fire with water. This was a very risky proposition, as molten metal oxidizes when in contact with water. The oxidation would create copious amounts of free hydrogen in the highly heated environment, possibly creating an explosion upon mixing with incoming air.
The workers used scaffolding poles to direct their hoses into fuel channels about a meter above the heart of the fire. As the cooling and ventilating air were shut off, Tuohy ordered the evacuation of everyone except himself and the fire chief. Tuohy scaled the reactor shielding one final time and ordered the water turned on. As the hoses sprayed the charge face, he listened carefully for any signs of a hydrogen reaction as the hoses sprayed the graphite core. Several more times he scaled up and down the reactor and reported how the flames slowly died away, "I went up to check several times until I was satisfied that the fire was out. I did stand to one side, sort of hopefully, but if you’re staring straight at the core of a shut down reactor you’re going to get quite a bit of radiation."
Though some radiation was leaked over the countryside, it didn't lead to any immediate death or injury to any of the reactor staff or members of the surrounding community. Reactor Manager Tom Tuohy-- thought to have been exposed to the most radiation during the event-- is now in his mid-80s and is living with his wife in the USA. One study conducted in 1987 estimated that as many as thirty-three people may eventually die from cancers as a result of this accident, though the Medical Research Council Committee concluded that "it is in the highest degree unlikely that any harm has been done to the health of anybody, whether a worker in the Windscale plant or a member of the general public." In contrast, Chernobyl caused forty-seven immediate deaths and as many as 9,000 may die from related cancer.
Today, some areas of Cumbria still prompt a few clicks from Geiger counters due to lingering caesium-137 isotopes. While the Windscale reactors have been in the process of being decommissioned since the 1980s, the core of Windscale Pile 1 still contains roughly fifteen tons of warm and highly radioactive uranium, and the cleanup is not expected to finish until 2060.
Ultimately the unnecessary incident could have been avoided with a bit of knowledge from the Manhattan project. Had the American government opted to share the nuclear knowledge which the British had helped to gain, the mishap could have been avoided altogether. Fortunately the foresight of Sir John Cockcroft and the valor of men like Tom Tuohy and Tom Hughes prevented this minor disaster from flaring into a national catastrophe.
Suggested by Steve Shirt